first I would get a supervisor involved. Maybe show on a sample of shipments that back up his unusual profitability. I would also get them to back up the 40% number with demographics. I think you are in a rural area, and as such your drivers may work for 1/2 the amount a NYC area driver might. Is the owner one of the drivers and only drawing a small salary from his new business? What is he hauling, I may be wrong in this but does BMW pay more than KIA to haul their cars? Many issues to fight with. I would also be a bit indigent that he is accusing the client of tax fraud by paying off the books. [even if true], never hurts to get your back up a bit.
There are 3 auditors in my area and probably 2 times a week one is in our office since we have so many payrolls. One was so nasty to staff that after his 4th time I called the company and said he is no longer allowed in our offices, they can switch the auditor or we can change insurance companies. They switched auditors. I know this isn't an answer to your issue but after you get done with the above audit, keep in mind that if someone is using arbitrary numbers to raise the premium, you can get rid of him.
A war story that I love: My client owner/manager of a lot of residential real estate in NYC. the agent wanted to add the value of the supers apartment to the income based on FMV. Well in many nyc buildings, the apartment designated as a supers apartment is a basement apt which doesn't get an official C of O. thus the apt have no FMV for anyone except a super. After the agent did research he agreed with me and we purely negotiated to charge a studio apt rate to each super. Cost my client a minimal amount, didn't have to involved in appeals and set a president for all future audits. We figured out that since some supers with families actually get a "real" apt this would all average out. My point is you can negotiate some issues with a reasonable agent.